The present invention relates generally to methods of forming tools from alloy steels useful in severe cold forming.
Cold forming is a highly economical and low cost method of manufacturing articles of simple to complex shape. One of the major constraints in the cold forming process is the material being formed, since the material must be able to undergo deformation without cracking or splitting in order successfully to manufacture the cold-formed part. The material properties are less stringent when the part being formed requires only limited deformation, but as the cold-formed part becomes more complex in geometry or requires more extensive deformation, the basic material properties must permit extensive flow of the metal to allow manufacture by the cold-forming process.
When the material utilized is steel, good formability, but low finished strength is provided by the use of a steel such as AISI 1008, a killed, low carbon, unalloyed steel, that possesses the ability to flow or be moved into complex shapes without difficulity. As the carbon content increases, to AISI 1021 for instance, the formability by cold forming is reduced, but a higher strength final part is produced. For complex, extensively formed parts, the carbon level in the steel is limited to approximately 0.35% by weight.
While increasing the carbon content of the steel produces higher strength cold-formed components, it is often found that spheroidized annealing is required sufficiently to soften the steel so that it can be adequately formed. Dependent upon the service requirements of the cold-formed component, the medium carbon steels are frequently heat treated to further enhance the mechanical properties of the finished component. Thus, the strength level and physical properties required of a specific component dictates a specific steel composition to be used, and whether it is to be heat treated. Consequently, it is often found that components which could be easily cold formed have service demands that require a steel composition that cannot be sufficiently cold formed to make the part. For example, a socket wrench as illustrated in FIG. 6 of the drawings may be easily formed from AISI 1008, may be produced without difficulty from AISI 1021, and may be produced with considerable difficulty from AISI 1035. Further, the socket wrench may not be able to be economically produced at all from a medium carbon alloy steel, such as AISI 8630, AISI 4140, or the like. Yet, the AISI 1008 socket wrench is of such low strength as to be of no use at all, while the AISI 1021 or the AISI 1035 socket wrenches may be adequate in medium service requirement areas, if heat treated. For a truly high performance socket wrench, one which will undergo repeated impact-loading service as found in professional service automotive garages, an alloy steel must be used. Nickel in the alloy steels markedly increases the steels impact performance, and hence a socket wrench made from AISI 8630 steel would outperform the other compositions referred to above.
However, medium carbon alloy steels, such as AISI 8630, do not have sufficient formability to economically produce components with severe deformation requirements, such as a socket wrench cold formed from a solid slug of such a steel. "Formability" results from a suitable combination of yield strength, tensile strength, elongation and reduction of area, such that the steel may be extensively formed without cracking or splitting. It must also be formable at sufficiently low pressure such that the cold-forming tool life is economical. In certain components requiring extreme deformation, the work-hardening rate of the steel is also of importance. Materials with high work-hardening rates require increasing pressures to be progressively deformed to a larger degree. Frequently, the forming operations must incorporate an intermediate annealing step if the component is to be successfully produced, a procedure which is highly uneconomical.
Poor formability of alloy steels as compared to carbon steels may be demonstrated from a comparison of the mechanical properties which contribute to formability. At equal carbon levels, a comparison can be made between AISI 8630 alloy steel and AISI 1030 carbon steel as follows:
______________________________________ Mechanical Property 8630 1030 ______________________________________ Tensile Strength, PSI 87,750 67,000 Yield Strength, PSI 54,000 49,000 Elongation, % 29 31 Reduction of Area, % 59 58 ______________________________________
While yield strength of AISI 8630 alloy steel is only 10% higher than that of AISI 1030 carbon steel, the tensile strength thereof is 31% higher and the elongation is slightly less. The relation of yield strength to tensile strength is an indication of the rate of work hardening, whereby it is seen that AISI 8630 alloy steel has greater deformation resistance and a greater work-hardening rate, coupled with lower elongation before fracture. AISI 8630 alloy steel may not be economically formed because of logarithmic reduction in tool life in proportion to the materials' increased tensile strength as compared to AISI 1030 carbon steel, and therefore, AISI 8630 alloy steel could not replace AISI 1030 carbon steel in the production of cold-formed components without seriously adversely affecting the economy of the process.
Summarizing, if the desired head hardened properties of AISI 8630 alloy steel are to be available in the products formed thereby, such as a socket wrench, the socket wrench must be machined at a rate of about 200 pieces per hour with a material loss amounting to 50% to 60% of the initial starting material. Alternatively, the AISI 8630 alloy steel could be cold formed only if multiple step forming processes were utilized with repeated annealing and forming steps performed sequentially. This method of forming socket wrenches is highly uneconomical.
The alloy steel used in the present invention is essentially a low carbon steel including nickel, chromium and molybdenum as alloying agents. The superior properties of the alloy tool for severe cold forming purposes reside in providing low controlled amounts of manganese, silicon and boron in the alloy steel.
Alloy steels containing manganese and silicon, but in higher or lower concentrations thereof are disclosed in the following U.S. Pat. Nos.: 2,737,455 granted Mar. 6, 1956 to H. W. Kirkby and C. Sykes; 3,093,519 granted June 11, 1963 to R. F. Decker, A. J. Goldman and J. T. Eash; 3,364,013 granted Jan. 16, 1968 to R. L. Caton; 3,396,013 granted Aug. 6, 1968 to J. R. Mihalisin, 3,418,110 granted Dec. 24, 1968 to S. Goda, I. Kimura and H. Masumoto; 3,615,370 granted Oct. 26, 1971 to K. A. Ridal and J. McCann; and 4,076,525 granted Feb. 28, 1978 to C. D. Little and P. M. Machmeier. Certain of these prior alloy steels contain too little manganese for the purposes of the present invention, these including U.S. Pat. Nos. 3,093,519 and 3,418,110. Others of these prior alloy steels contain too much silicon, including the alloy steels disclosed in U.S. Pat. Nos. 2,737,455; 3,364,013; 3,396,013; 3,418,110 and 3,615,370.
The lack of synergistic amounts of manganese, silicon and boron in the prior art alloy steels causes those steels to have a ductility and a rate of work hardening which render those alloy steels unsuitable for extreme cold forming of articles, specifically tools, therefrom. Such severe cold forming requires a superior ductility and a low rate of work hardening of the annealed alloy steel which is not achieved by the alloy steels of this prior art.
There also is disclosed in U.S. Pat. No. 4,061,013 granted Dec. 6, 1977 to J. Kuc and A. Kuc an alloy steel and a method of cold forming that alloy steel to provide a forged tool. The alloy steel of this prior patent does not possess the superior ductility and low rate of work hardening of the alloy steel used in the present invention. This results in large measure from the increased amount of manganese in this prior art alloy steel and the lack of control of the silicon content thereof.